1. Field of the Invention
The invention relates generally to seismic energy vibrators and, more particularly, but not by way of limitation, it relates to shear/compressional wave sources employing a traveling wave tube for coupling acoustic energy into the earth at soft, wet or marshy terrain.
2. Description of the Prior Art
Prior teachings are best characterized by the U.S. Pat. No. 3,365,019 in the name of Bays and entitled "Seismic Vibrator for Marshland and Submarine Use". This early vibrator teaching was directed to compressional wave energy generation and it used a cupshaped coupling or foot member to engage a vibratory source into marshland surfaces. Conical outer surfaces of the foot member served to maintain continual energy-coupling engagement through a vibratory sequence as the vibrator sank progressively into the earth. It was necessary to move the vibrator to more firm footing periodically. This teaching, of course, was directed solely to compressional wave generation and to Applicant's knowledge there has been no similar effort directed to generation of shear waves. Existing shear wave generator teachings, both vibratory and impulsive, can only couple to hard surfaces and produce linearly polarized shear waves. Thus, the only existing alternative is the use of mode-converted shear wave data when such soft terrain areas are encountered.